It is known to prepare purified synthesis gas from a carbonaceous feedstock by first performing a partial oxidation, also referred to as gasification, of the feedstock followed by a scrubbing step, a CO shift step and a CO2 and H2S removal step. Such a process is described in Gasification Technology Counsel (GTC 2004) Conference paper called “Synthesis Gas Purification in Gasification to Ammonia/Urea Complex” of John Y Mak and others (http://www.gasification.org/Presentations/2004.htm). In FIG. 5 of this paper an integrated process is described wherein a coal slurry feed is gasified in a so-called quench gasification process to obtain a raw synthesis gas. After scrubbing and a CO shift step of the raw synthesis gas CO2 and H2S and other acid gasses are removed to obtain a treated synthesis gas. The CO2 and H2S removal is performed by the well-known Rectisol process of Lurgi AG and Linde AG. In the Rectisol process CO2 and H2S and other acid gasses are removed by contacting the synthesis gas with methanol at cryogenic temperatures.
In the Rectisol process as shown in FIG. 6 of the above publication the synthesis gas is first mixed with methanol prior to reducing the temperature of said mixture. By cooling a liquid methanol-water mixture is formed, which is separated from the gaseous synthesis gas in so-called knock-out drums. The cooled synthesis gas is subsequently contacted with cold methanol in the so-called methanol scrubber to obtain a treated synthesis gas and a rich methanol, comprising CO2, H2S and other acid gasses CO2, H2S and other acid gasses are removed from the rich methanol and the regenerated methanol is reused in the methanol scrubber.
The methanol as present in the water-methanol mixture is recovered in the so-called methanol/water fractionator and reused in the methanol scrubber.
An issue with the above process is that part of the methanol inventory has to be removed from the circulating methanol to avoid a build-up of certain contaminants, such as for example ammonia, cyanide, unidentified sulphur species and trace metals.
It would be an advancement in the art to provide a more efficient process, which avoids the build up of such contaminants in the CO2 and H2S removal step.